I still can't figure this one out:

http://fmforums.com/forum/showtopic.php?tid/210459/

If I have a fresh, clean, virgin record that I have not edited via php, I can successfully edit it via php once. If I try to edit it again via php, Filemaker returns "Unknown error." Even if I log out and back in. Even if I go in and edit the record from within Filemaker, it still won't let me edit it again via php. Is there something that should come after the commit() to free the record up for editing?

No, there is nothing to commit if you press "Submit" button on browser page.

If such happen - something is wrong, I suppose - check PHP editrecord.php (if PHP is made by PHP Assistant).

Other thing - somebody was "sitting" in record, don't open the same record via PHP and FM at the same time.

From my experience - PHP is sensitive and must be correct.

I don't know if it matters, but I'm not doing it with a "Submit" button; I'm doing it by a link, i.e., one page lists the records, and if you click on one of them, it passes the record i.d. in the URL and goes to a page that finds that record and then performs the set fields and commit on the found record.

If someone's in the record, it returns "Error: 301 - Record is in use by another user."

Doh! It was this: on the Server Admin Console, configuring Web Publishing Engine, on the PHP tab, there's a checkbox for "record data pre-validation." I unchecked it, and now it works. I don't know what it validates, as none of the fields I'm editing have any validations in Filemaker.